I also can't imagine there is much demand for IMAP only email migration services these days.
That said, I was saved/pleased by Eudora2Unix [0], one of those projects that represents a very long slow burn of successive people with a similar niche struggle. You might think Thunderbird would have an import tool for that, but it's been so many years it didn't survive...
This is the way projects used to be, and surprisingly excellent ones still are.
The amount of knowledge built into this is incredible:
https://imapsync.lamiral.info/S/news.shtml
// imapsync did 14M to 21M mailboxes transfers per month in 2024, or 0.22% of ALL email traffic
techstuff123•2h ago
It supports large mailboxes, preserves full data integrity, requires no setup, and works with any IMAP-compatible email service.